1. Field of the Invention
This invention relates to a method of producing polyurethanes, especially for coating purposes.
2. Description of the Prior Art
In the conventional processes for producing polyurethanes in the form of coatings using blocked or capped isocyanate compounds, baking at high temperatures, at about 200.degree. C., for instance, is required. Therefore, in coating relatively thin steel sheets, there arise such problems as deformation of the steel sheets themselves, melting of solders and excessive energy consumption.
For solution of these problems, it is strongly desired to lower the deblocking temperatures of the blocked isocyanate compounds, and so far there have been proposed as catalysts for lowering the deblocking temperatures 1,1,3,3-tetrabutyl-1,3-diacetoxydistannoxane, 1,1,3,3-tetrabutyl-1,3-diphenoxydistannoxane, 1,1,3,3-tetrabutyl-1,3-dibenzyloxydistannoxane, dibutyltin oxide, dibutyltin dilaurate and tetrabutyltin, for instance, (see, for example, Japanese Patent Publication No. 18877/1969, Japanese Patent Applications laid open (Kokai) under Nos. 42528/1977, 138434/1978 and 138435/1978, and U.S. Pat. No. 4,101,486). However, the effects of these compounds are still unsatisfactory. More efficient catalysts are demanded.